The present invention relates to a torque sensor and more particularly to a torque sensor having coils for detecting torque and fit for use in a vehicular power steering system in which irregularities resulting from the inadequate connection of the coils and the like are detected.
With respect to the prior art, U.S. Pat. No. 5,406,834 discloses a type of torque sensor mentioned above.
In the conventional apparatus, the torque sensor is designed to detect torque by first applying oscillating voltage to a torque detecting coil and a temperature compensating coil, and differentially amplifying the voltages detected from these coils. A reference voltage generated by dividing resistance voltage is applied in the forward direction via diodes to one of the differential inputs. In this torque sensor, when the detection voltage becomes lower than the reference voltage as the oscillating voltage applied to the coils dies out because an oscillation circuit malfunctions, since one of the differential inputs is maintained at the reference voltage, the balance between the differential inputs is broken. Then the output subjected to the differential amplification is forced to vary from a predetermined range, whereby the malfunction of the oscillating circuit can be detected from the output thus subjected to the differential amplification. By merely differentially amplifying and outputting the detection voltage from the coils, because both the differential inputs lower when the oscillating voltage dies out, the output after the differential amplification is not usable to determine the irregularity of the oscillating voltage.
The dying out of the oscillating voltage resulting from the malfunction of the oscillation circuit has made been made detected with certainty by means of the outputs of the differential amplifier circuits in the conventional torque sensor.
However, there are various kinds of irregularities of torque sensors, and some of the irregularities other than those resulting from the malfunction of the oscillation circuit still remain undetected in the apparatus where only the output is subjected to differential amplification because both the differential inputs vary together. For example, there arise irregularities in cases where a connector provided between the coils and the oscillation circuit is disconnected, where the lead wire of the coil is broken partly because a coil yoke holding the coil is turned, and where the lead wire of the coil is broken when the coils are loaded. In these cases, both coils are not connected, which is equivalent to a case where infinite inductance is electrically connected. When the oscillating voltage in this condition is supplied, it is directly detected and the detection voltages are caused to have a great value; that is, no output irregularity appears even when the pair of detection voltages are differentially amplified. Since the oscillating voltage is being supplied, the irregularity is not detected by the conventional torque sensor which operates on the basis of the dying out of the oscillating voltage. Therefore, the problem is that the control circuit for receiving the outputs of the differential amplifier circuits and the like are not capable of deciding whether or not such an irregularity exists in the torque sensor by only monitoring the outputs of the differential amplifier circuits